Vibrational level structure in the S01Ag and S1 1B3u states of pyrene was investigated through analysis of fluorescence excitation spectra and dispersed fluorescence spectra for single vibronic level excitation in a supersonic jet and through referring to the results of ab initio theoretical calculation. The vibrational energies are very similar in the both states. We found broad spectral feature in the dispersed fluorescence spectrum for single vibronic level excitation with an excess energy of 730 cm-1. This indicates that intramolecular vibrational redistribution efficiently occurs at small amounts of excess energy in the S1 1B3u state of pyrene. We have also observed a rotationally resolved ultrahigh-resolution spectrum of the 0 00 band. Rotational constants have been determined and it has been shown that the pyrene molecule is planar in both the S0 and S1 states, and that its geometrical structure does not change significantly upon electronic excitation. Broadening of rotational lines with the magnetic field by the Zeeman splitting of MJ levels was very small, indicating that intersystem crossing to the triplet state is minimal. The long fluorescence lifetime indicates that internal conversion to the S 0 state is also slow. We conclude that the similarity of pyrene's molecular structure and potential energy curve in its S0 and S 1 states is the main cause of the slow radiationless transitions.